1. Field of the Invention
This invention relates to improved fuel additive compositions. The fuel additives of the invention provide improved low temperature flow and filterability to distillate fuels, such as diesel fuels, and are substantially non-discoloring and non-corrosive. Distillate fuels containing the fuel additive compositions are also provided.
2. Description of the Prior Art
Distillate fuels such as diesel fuels tend to exhibit reduced flow at low temperatures due in part to formation of waxy solids in the fuel. The reduced flow of the distillate fuel affects transport and use of the distillate fuels in refinery operations and internal combustion engine. This is a particular problem during the winter months and especially in northern regions where the distillates are frequently exposed to temperatures at which solid formation begins to occur in the fuel, generally known as the cloud point (ASTM D 2500) or wax appearance point (ASTM D 3117). The formation of waxy solids in the fuel will in time essentially prevent the ability of the fuel to flow, thus plugging transport lines such as refinery piping and engine fuel supply lines. Under low temperature conditions during consumption of the distillate fuel, as in a diesel engine, wax precipitation and gelation can cause the engine fuel filters to plug which can be simulated in the laboratory using tests such as the low-temperature flow test (LTFT). This test, ASTM Designation D 4539-98, estimates the filterability of diesel fuels in automotive equipment at low temperatures. For the test, fuel samples are cooled at a prescribed rate and at the desired temperature and each 1xc2x0 C. interval thereafter, a specimen of the fuel is filtered through a 17 xcexcm screen utilizing a vacuum system. The minimum LTFT pass temperature is the lowest temperature at which the prescribed volume of fuel (180 ml) can be filtered in 60 seconds or less. Alternatively, a single fuel specimen may be cooled in the above-described manner and tested at a specified temperature to determine whether it passes or fails at that temperature.
As used herein, distillate fuels encompass a range of fuel types, typically including but not limited to kerosene, intermediate or middle distillates, lower volatility distillate gas oils and higher viscosity distillates. Grades encompassed by the term include Grades No. 1-D, 2-D and 4-D for diesel fuels as defined in ASTM D975, incorporated herein by reference. The distillate fuels are useful in a range of applications, including use in automotive diesel engines and in non-automotive applications under both varying and relatively constant speed and load conditions.
Distillate fuels are comprised of a mixture of hydrocarbons including normal and branched-chain paraffins, olefins, aromatics and other polar and non-polar compounds, and cold flow behavior is a function of the relative proportion of these various hydrocarbon components. Normal paraffins typically have the lowest solubility and therefore tend to be the first solids to separate from the fuel as the temperature is decreased. At first, individual paraffin crystals will appear but as more crystals form they will ultimately create a gel-like network which inhibits flow. The compositional makeup of fuels can vary widely depending on the crude oil source and how deeply the refiner cuts into the crude oil. With mounting pressure on refiners to increase production of distillate fuels, they are increasingly producing fuels with amounts and types of hydrocarbon components which render the fuels unresponsive to additives heretofore capable of imparting acceptable cold flow properties to the fuels. These fuels are referred to within the industry as xe2x80x9chard-to-treatxe2x80x9d fuels.
A number of compositional features can contribute to the unresponsiveness of hard-to-treat fuels to flow additives, including one or more of the following: a narrow molecular weight distribution of waxes; the virtual absence of high molecular weight waxes; inordinately large amounts of very high molecular weight waxes; a higher percentage (total) of wax; and a higher average carbon number for the normal paraffin component. While it is difficult to generate a single set of parameters which define hard-to-treat fuels, they are typically characterized by one or more of the following distillation parameters (as determined by test method ASTM D 86 incorporated herein by reference): the temperature differential between the 20% distilled and 90% distilled fractions; the temperature differential between the 90% distilled fraction and the final boiling point; and the final boiling point.
Useful cold flow improvers for distillate fuels, including hard-to-treat fuels, are disclosed in U.S. Pat. No. 6,203,583. The cold flow additives of the invention are a combination of an ethylene/vinyl acetate/isobutylene copolymer with one or more of a maleic anhydride/xcex1-olefin copolymer component, a polyimide component and an alkylphenol component. Similar compositions useful as wax anti-settling agents and cloud point depressants for distillate fuels are disclosed in U.S. Pat. Nos. 6,206,939 and 6,143,043, respectively.
While certain of the above-mentioned additives do improve cold flow properties of distillate fuels to some extent, there continues to be a need for additives which exhibit enhanced performance, particularly for hard-to-treat fuels. For example, there is an ongoing need for cold flow improver additives which do not interact with the distillate fuel or other additives commonly contained therein and, in turn, discolor the fuel or cause the formation of undesirable deposits upon storage. Cold flow improver additives which tend to discolor distillate fuels, either by interaction with other additives, e.g., stabilizers, or by other means, can interfere with or mask dyes which are added to differentiate fuels, such as dyes added to tax-exempt off-road fuel. Accordingly, it would be highly advantageous if cold flow improver fuel additive compositions were available which provided both improved cold flow performance and stability for distillate fuels. It would be even more useful if the fuel additives were substantially non-acidic to prevent corrosion of metal storage tanks and transfer lines.
The present invention relates to improved fuel additive compositions and to distillate fuels, including hard-to-treat distillate fuels, containing said additives. The additives of the invention impart improved low temperature flow and filterability to distillate fuels and also serve to stabilize the fuels against the development of undesirable color or deposits upon storage. The fuel additives are a combination of an olefin/vinyl carboxylate polymer with first and second polyimides of specific structure. More specifically, the additives comprise (a) an olefin/vinyl carboxylate polymer selected from the group consisting of ethylene/vinyl acetate copolymers; ethylene/vinyl acetate/isobutylene terpolymers and mixtures thereof; (b) a first polyimide corresponding to the general formula 
where R1 is an alkyl group with an average carbon number of 22 to 26 carbon atoms and n1 is from about 1.5 to 8; and (c) a second polyimide corresponding to the general formula 
where R2 is an alkyl group with an average carbon number greater than 30 and n2 is from about 1.5 to 8; said first polyimide and said second polyimide present at a weight ratio of 1:5 to 5:1 and the weight ratio of olefin/vinyl carboxylate polymer to the combined weight of said first and second polyimides ranging from 4:1 to 1:4.
Improved distillate fuel compositions containing 100 to 5000 ppm of the above-defined additives are also provided.
In accordance with the present invention, fuel additive compositions are provided which impart significantly improved cold flow properties, i.e., flowability and filterability, to distillate fuels and particularly hard-to-treat distillate fuels. Additionally, the fuel additive compositions of the invention do not adversely affect fuel stability.
The additive compositions of the invention are comprised of an olefin/vinyl carboxylate polymer and a mixture of a first polyimide and a second polyimide, said polyimides having repeating units corresponding to the general structure: 
but differing in the number of carbon atoms in the pendant R group. The olefin/vinyl carboxylate polymer and first and second polyimides are present within prescribed weight ratio limits.
Useful olefin/vinyl carboxylate polymers include ethylene/vinyl acetate copolymers (EVA) and ethylene/vinyl acetate/isobutylene terpolymers (EVAiB) or combinations thereof. The EVA and EVAiB polymers will have weight average molecular weights in the range of about 1,500 to about 18,000, number average molecular weights in the range of about 400 to about 3,000 and a ratio of weight average molecular weight to number average molecular weight from about 1.5 to about 6. Preferably the weight average molecular weight ranges from about 3,000 to about 12, 000 and the number average molecular weight ranges from about 1,500 to about 2,500. The EVA and EVAiB polymers have Brookfield viscosities in the range of about 100 to about 300 centipoise (cP) at 140xc2x0 C. More typically the Brookfield viscosity is in the range of about 100 to about 200 centipoise. Vinyl acetate contents will range from about 25 to about 55 weight percent. Preferably the vinyl acetate content ranges from about 25 to about 45 weight percent and, even more preferably, from about 27 to about 38 weight percent. The branching index is from 2 to 15 and, more preferably, 5 to 10. The EVA copolymers and terpolymers are produced in accordance with known procedures. For example, the EVAiB copolymers are described in U.S. Pat. Nos. 5,256,166 and 5,681,359 which are incorporated herein by reference.
A first and second polyimide are combined with the EVA, EVAiB or EVA/EVAiB blend to obtain the improved fuel additive compositions of the invention. The polyimides correspond to the general formula: 
where R represents an alkyl moiety and n is the number of repeating units of the polyimide. The first and second polyimides utilized to obtain the improved compositions of the invention have different alkyl substituents, which are hereinafter respectively designated as R1 and R2. The number of repeating units of the first and second polyimide may be the same or different and are hereinafter respectively designated n1 and n2.
The alkyl substituent (R1) for the first polyimide will be an alkyl group with an average carbon number of 22 to 26 carbon atoms. Preferably 60% or more of the alkyl substituents of the first polyimide will have 22 to 26 carbon atoms. Most preferably, the alkyl substituent R1 of the first polyimide is comprised of at least 70% C22-26 alkyl substituents. The number of repeating units (n1) of the first polyimide will be from about 1.5 to 8 and the number average molecular weight (Mn) will range from about 600 to 8000. Weight average molecular weights (Mw) range from about 1500 to 15000.
The second polyimide will have an alkyl substituent (R2) with an average carbon number substantially higher than that of the first polyimide. R2 for the second polyimide will have an average carbon number greater than 30. Preferably 60% or more of the alkyl substituents of the second polyimide will have 30 to 36 carbon atoms. Most preferably at least 70% of the R2 alkyl substituents will be C30-36 alkyl substituents. The number of repeating units (n2) for the second polyimide will be from about 1.5 to 8 and the number average molecular weight will range from about 650 to 9500. Weight average molecular weights for the second polyimide are from about 2000 to 21000.
Both polyimides are produced using known procedures wherein an xcex1-olefin having the requisite number of carbon atoms is copolymerized with a substantially equimolar amount of maleic anhydride by means of free radical catalysis and in a subsequent reaction forming the corresponding polyimide by neutralizing with ammonia at an elevated temperature. xcex1-Olefins used in making the xcex1-olefin/maleic anhydride copolymer precursors are mixtures of xcex1-olefins having a distribution of carbon numbers so as to obtain the different alkyl substituents for the first and second polyimides. For example, to produce a first polyimide wherein 60% or more of the alkyl groups (R1) have 22 to 26 carbon atoms, an xcex1-olefin wherein 60% or more of the olefins contain 24 to 28 carbon atoms would be reacted with maleic anhydride to form the xcex1-olefin/maleic anhydride precursor.
Effective fuel additive compositions are obtained by combining the EVA copolymer, EVAiB terpolymer or combination thereof and the first and second polyimides at a weight ratio of from 4:1 to 1:4 and, more preferably, from 2:1 to 1:2. The polyimide component in the foregoing weight ratios represents the total weight of both the first and second polyimides. The first and second polyimides are utilized at weight ratios from 1:5 to 5:1 and, more preferably, from 1:2.5 to 2.5:1. In one highly useful embodiment, 2 parts EVA, EVAiB, or mixture thereof are combined with 1 part first polyimide and 1 part second polyimide.
The fuel additive compositions of the invention are typically added to the distillate fuels at levels from about 100 ppm up to about 5000 ppm. While higher levels of additive can be used, any additional benefit obtained does not usually justify the additional cost. Especially useful additive levels are 150 to 3000 ppm and, more preferably, 200 to 2500 ppm.
The following detailed examples illustrate the practice of the invention in its most preferred form, thereby enabling a person of ordinary skill in the art to practice the invention. The principles of this invention, its operating parameters and other obvious modifications thereof, will be understood in view of the following detailed procedure. All parts and percentages in the examples are on a weight basis unless otherwise indicated.
To demonstrate the improved cold flow performance of the additive compositions, the additives were combined with various diesel fuels at weight concentrations ranging from 125 to 1000 ppm. All of the fuel formulations were prepared by the addition of a concentrate containing 10% of the additive composition (2 parts ethylene/vinyl carboxylate copolymer or terpolymer, 1 part first polyimide and 1 part second polyimide) in a mixed aromatic solvent (Aromatic 100). The desired concentration of additive in the fuel was obtained by varying the amount of concentrate added to the fuel.
Three olefin/vinyl carboxylate (OVC) polymers were utilized to prepare the various fuel additive compositions utilized in the examples and they are identified in Table 1. Polyimides used for the fuel additive compositions are identified in Table 2. Brookfield viscosities for the first polyimides (P1) and second polyimides (P2) used are provided. Brookfield viscosities for the polyimides were determined using hydrocarbon (Aromatic 100) solutions containing 35 weight % polyimide. The acid numbers for the xcex1-olefin/maleic anhydride from which each of the polyimides was derived are included in the table.
Various fuels were used in the examples to demonstrate the improved performance of the additive compositions of the invention. The fuels are listed in Table 3 with distillation data for each determined in accordance with ASTM D 86. The data include the initial boiling point (IBP), final boiling point (FBP) and the temperature at which specific volume percentages of the fuel have been recovered from the original pot contents at atmospheric pressure. All temperatures are in xc2x0 C.
Table 4 sets forth the distillation criteria generally utilized by the industry to characterize hard-to-treat fuels. This criteria utilizes the temperature difference between the 20% distilled and 90% distilled temperatures (90%-20%), the temperature difference between the 90% distilled temperature and final boiling point (FBP-90%) and the final boiling point. A 90%-20% temperature difference of about 100-120xc2x0 C. for middle distillate cut fuels is considered normal. A difference of about 70xc2x0-100xc2x0 C. is considered narrow and hard-to-treat and a difference of less than about 70xc2x0 C. is considered extremely narrow and very hard-to-treat. A FBP-90% temperature difference in the range of 25xc2x0 C. to about 35xc2x0 C. is considered normal. A difference of less than about 25xc2x0 C. is considered narrow and hard-to-treat. A difference of more than about 35xc2x0 C. is also considered hard-to-treat. A final boiling point below about 360xc2x0 C. or above about 380xc2x0 C. is considered hard-to-treat. Additional disclosure on hard-to-treat fuels is found in U.S. Pat. No. 5,681,359, incorporated herein by reference. From an examination of the distillation data provided in Table 4, it will be observed that all of the fuels employed for the examples satisfy one or more of the above-described criteria and would therefore be considered hard-to-treat fuels.